Field of the Invention
One or more embodiments of the present invention relates to a battery electrode or separator surface protective agent composition, a method for protecting a battery electrode or separator surface using the composition, a battery electrode or separator coated with the composition, and a battery comprising the battery electrode or separator. The battery comprising the battery electrode or separator is advantageous not only in that it has high heat resistance, low internal resistance, and excellent charge/discharge cycle characteristics as well as large charge/discharge capacity, but also in that even when the battery is charged and discharged in many cycles repeatedly for a long term, the deterioration of the active material layer is small, achieving an increased life of the battery.
Background Art
Some of lithium-ion primary battery and secondary battery which are lightweight and which have a high voltage and a large capacity, calcium-ion primary battery and secondary battery, magnesium-ion primary battery and secondary battery, sodium-ion primary battery and secondary battery, and an electrical double layer capacitor having excellent charge and discharge rate characteristics have been put into practical use as a power source for mobile electric devices, such as a cell phone and a laptop computer, and power tools, such as an electric tool and a vehicle. However, a conventional battery has problems in that the safety is poor due to low heat resistance or crush resistance, and in that conductive foreign matter mixed into the battery in the stage of production penetrates the separator to cause short-circuiting. In addition, the conventional battery has a high internal resistance, practically unsatisfactory high-rate charge/discharge characteristics, and unsatisfactory charge/discharge capacity, and, when the battery is used for a long term, the active material layer markedly deteriorates.
One of the reasons why satisfactory safety of the battery cannot be provided as mentioned above resides in that with respect to the heat generation by the occurrence of short-circuiting due to the breakdown of the insulation by the separator caused by, e.g., mixing of conductive foreign matter, the generation of dendrite, or breakage of the battery, the method for preventing the runaway heat generation from rapidly proceeding in the battery is inappropriate.
As a method for solving the above problem, a method has been proposed in which a coating layer having high heat resistance, which has insulation with respect to the electronic conductivity but has ionic conductivity, is formed on a battery electrode and/or separator surface so that, even when runaway heat generation causes the separator to thermally degrade, the coating layer prevents the positive and negative electrodes from touching to cause short-circuiting (patent document 1). The above coating layer acts as a layer which suppresses the generation of dendrite or retains the electrolytic solution, and the coating layer serves as an ion supply source to reduce the internal resistance, contributing to the improvement of the high-rate discharge characteristics. Further, the coating layer makes the electrode surface uniform, and therefore local acceleration of the deterioration of the active material layer caused by the concentration of an electrode reaction due to the uneven surface of the electrode can be suppressed, and thus an effect of preventing the deterioration of the active material layer caused when the battery is used for a long term can be obtained. However, in this method, the coating layer has poor adhesion to the electrode surface and further has low ionic conductivity, and therefore, when the method is applied to a battery for use as power supply for vehicle and others, the resultant battery has high internal resistance so that it cannot achieve high-rate charge/discharge cycle characteristics, and particularly, the high internal resistance causes charge and discharge loss, making it difficult to achieve satisfactory running distance. Furthermore, the batter has no satisfactory effect of relaxation of expansion and shrinkage stress of a material constituting the battery, such as an active material, caused due to charging and discharging, leading to a problem in that the adhesion force, mechanical strength and others are lowered.
Further, there is a coating layer of a composite type in the form of an emulsion of, e.g., an elastomer having high ionic conductivity, and this coating layer has both high ionic conductivity derived from an elastomer constituting the island portion and a mechanical strength derived from a polymer constituting the sea portion, and therefore exhibits excellent ionic conductivity and excellent mechanical strength as well as excellent stress relaxation effect (patent document 2). However, in this method, the coating layer has low ionic conductivity, and therefore, when the method is applied to a battery for use as power supply for vehicle and others, the resultant battery has high internal resistance so that it cannot achieve high-rate charge/discharge cycle characteristics, and particularly, the high internal resistance causes charge and discharge loss, making it difficult to achieve satisfactory running distance.
Furthermore, there is a coating layer of a type having incorporated inorganic particles, such as a ceramic, and this coating layer conducts ions through a polar group present on the ceramic surface, and hence exhibits excellent ionic conductivity and excellent mechanical strength by virtue of a reinforcing effect of the ceramic (patent document 3). However, there is a problem in that a binder having poor ionic conductivity covers the individual particles to clog voids between the particles, causing the ionic conductivity to be poor.
Moreover, a method has been proposed in which the active material and conductive agent are bonded using a binder in a particulate form, and the binder covers the active material and conductive agent to prevent the lowering of the ionic conductivity. However, this method has a problem in that much of the binder does not constitute a continuous layer and further many voids are formed between the active material and conductive agent, lowering the mechanical strength (patent document 4).